A Luminal Calcium Sensing Mutation of the Cardiac Ryanodine Receptor Diminishes Calcium Waves and Stress -Induced Ventricular Tachycardias in Calsequestrin Null Mice

Abstract

Catecholaminergic polymorphic ventricular tachycardia (CPVT) is an inherited arrhythmogenic disease. Naturally occurring mutations in the cardiac ryanodine receptor (RyR2) and calsequestrin (CASQ2) have been linked to CPVT. The sarcoplasmic reticulum calcium content and the sensitivity of RyR2 to luminal calcium are two important factors determining the propensity for store overload induced spontaneous calcium release, also known as store-overload induced calcium release (SOICR), and SOICR-evoked VTs. We recently found that residue E4872 of RyR2 is critical for luminal calcium sensing. In the present study, we determined whether the RyR2-E4872Q mutation is capable of rescuing the VT phenotype of the CASQ2 null mice. We bred E4872Q+/- mice with CASQ2 null mice to produce CASQ2-/- or CASQ2+/- mice with or without the RyR2-E4872Q mutation. CASQ2-/- and CASQ2+/- mice without the RyR2-E4872Q mutation showed long lasting stress-induced VTs, whereas CASQ2-/- / E4872Q+/- and CASQ2+/- / E4872Q +/- displayed little or no VTs. Thus, the RyR2-E4872Q mutation nearly completely protected the CASQ2 null mice from CPVT. We next studied the impact of the RyR2-E4872Q mutation on calcium handling in intact hearts using linescan confocal calcium imaging. The properties of calcium transients did not differ significantly between CASQ2-/- hearts with or without the RyR2-E4872Q mutation at resting conditions. However, upon elevation of external calcium from 2mM to 7mM or adding 100nM isoproterenol, frequent spontaneous calcium waves occurred in CASQ2-/- hearts without the RyR2-E4872Q mutation, but not in CASQ2-/- / E4872Q+/- hearts. These observations indicate that targeting the luminal calcium sensor of RyR2 represents a new therapeutic strategy for suppressing CPVT and other calcium mediated cardiac arrhythmias. (Support by NIH R01HL75210)